1. Field of the Invention
This invention relates to a conductive material comprising a reaction-sintered body which incurs only a slight dimensional change in the sintering operation, and a process for producing such conductive material.
2. Description of Related Art
Highly heat resistant SiC, Si.sub.3 N.sub.4 and the like are generally known as engineering ceramics suited for use as structural material for engines, turbines and the like. Among the known techniques for sintering such materials are the so-called pressureless sintering method, pressure sintering method and reaction-sintering method. Among them, a pressureless sintering method and pressure sintering method carry a high risk of deformation and are poor in dimensional precision, with the possible rate of dimensional change at the time of sintering running as high as 15 to 20%, and highgrade techniques are required for these methods on the part of the workers. The high rate of dimensional change during sintering requires much time and costs for post-sintering work, and this has been one of the greatest obstructions to the popular use of engineering ceramics. As for the reaction-sintering method, on the other hand, it is known that the rate of dimensional change suffered at the time of sintering is small in comparison with the materials sintered by other methods, and it is only disclosed in Japanese Patent Kokai (Laid-Open) No. 58-140375 that the material is composed of a nitride of metallic Si powder and little is known about the conductivity of such material.
Si.sub.3 N.sub.4 binding material, which has been generally used as a refractory, is also a material which is expected to suffer little dimensional change in sintering. Such material is disclosed in Japanese Patent Kokai (Laid-Open) No. 58-88169, but nothing is told about conductivity. Further, the mechanical strength of this material is as low as about 50 MPa, which frustrates any expectation of its use as a structural material.
Lately, request is strong for the development of conductive ceramics useful as heat-resistant heater materials or conductors. Invention of ceramics with small electrical resistivity would contribute greatly to the improvement of performance of currently used products and would also pave the way for new and wider use of ceramics. In the conventional conductive ceramics, as for instance disclosed in Japanese Patent Kokai (Laid-Open) Nos. 50-84936 and 60-44990, a conductive compound is mixed with SiC or Si.sub.3 N.sub.4 and the mixture is hot-press sintered to solve the problem of electroconductivity. The hot press method, however, involves the problem of high production costs as it requires a vast amount of energy for sintering.
Further, Japanese Patent Kokai (Laid-Open) No. 60-60983 discloses a method for obtaining a conductive ceramic material by mixing a conductive compound with Si.sub.3 N.sub.4 and subjecting the mixture to pressureless sintering which is more advantageous in terms of energy than the hot press method. However, since this method utilizes a sintering aid, the rate of dimensional change at the time of sintering may rise up to 15-18%, posing the problem of intolerable deformation.
Japanese Patent Kokai (Laid-Open) No. 61-247662 discloses a sintered body comprising a Cr carbonitride obtained by reacting and sintering a Cr powder molding with CO in N.sub.2 gas at 1,500.degree. C., such sintered body being described as having a specific resistance at room temperature of more than 10.sup.-4 .OMEGA.cm. In this case, since a Cr powder molding is reacted and sintered in a CO-containing N.sub.2 gas, the produced sintered body differs in composition between the surface portion and the inside portion, that is, the obtained sintered body has different properties at its surface and in the inside, and thus there can not be obtained a sintered body with uniform quality.
As described above, there have been available no practical techniques for producing ceramics having excellent dimensional precision and capable of controlling electrical resistivity to a low level.
The aforementioned conventional techniques have been deficient in the rate of dimensional change at the time of sintering, electrical resistivity, mechanical strength, etc., and the use of the products as mechanical structural material or functional material has been limited.